1. Field of the Invention
The present invention relates to a current-detecting type dry-operative ion-selective electrode. More particularly, the present invention relates to make an electrochemical sensor dry-operative, which is used to measure quickly and simply the presence and an amount of a specific material in a sample to be assayed utilizing a polarization in a voltammeteric electrochemical analysis.
2. Description of the Related Art
In these years, a measurement using an electrochemical sensor has been widely used in the medical field.
For example, a potential-detecting type ion-selective electrode (hereinafter referred to as "ISE") is widely used as a safe and handy analytical element which requires no gas cylinder, in place of analysis of sodium or potassium ion by a conventional flame photometric detection or analysis of chloride ion by a coulometric titration. Recently, a disposable dry-operative plate form ISE is proposed to measure a concentration i of an electrolyte in a body fluid, and a portable exclusive system which performs the measurement more quickly and easily is commercialized.
Japanese Patent KOKAI Publication Nos. 287146/1990 and 287155/1990 disclose such disposable plate form ISE and the exclusive measuring equipment therefor.
In such ISE, a functional film is formed by covering a surface of a silver/silver chloride electrode with an electrolyte layer and a sensitive material which is reactive with a specific ion, and the functional film is contacted to the sample to be assayed to detect the presence of the specific ion in the sample from a potential response which is proportional to a logarithm of the ionic activity.
A current-detecting type ISE is a novel ion-selective electrode which can detect an ion (or electron) moving reaction at an interface between two immiscible electrolyte solutions, namely an oil-water interface by a method similar to conventional polarography or voltammetry. With this ISE, since the response of current is proportional directly to the ion concentration, very accurate measured values can be obtained in a limited concentration range of the specific material as in the body fluid. If an interfering ion is present, a curve due to the interfering ion appears on a voltamogram (a current-potential curve) in addition to the curve due to the target ion. When half-wave potentials of the both ions are separated by a suitable distance, the influence of the interfering ion can be corrected.
From another point of view, two or more kinds of ions can be quantitatively measured at the same time. Such is reported by J. Koryta, Electrochim. Acta, 33, 189 (1988) and M. Senda, et al, Electrochim. Acta, 36, 253 (1991).
However, since the potential response measured by the conventional potential-detecting type ISE is proportional to the logarithm of the ion activity, the potential-detecting type ISE is suitable for measuring the material in a wide range of concentration. However, an error in the measured value in the very narrow concentration range as in the body fluid cannot be neglected clinically.
Theoretically, .+-.1 mV error in the potential value will produce .+-.4% error in the case of the Nernst response of a monovalent ion. In fact, the error of about .+-.1 mV is unavoidable due to the influence of co-present material in the analysis, and such error will make a trouble when the accurate ion concentration should be known.
In contrast, the current-detecting type ISE is excellent in accuracy. However, many problems should be solved before its practical use. For example, a method for producing a probe type (cylindrical) current-detecting type ISE shown in FIG. 1 is proposed by T. Osakai et al, Anal. Sci., 3,521 (1987). This probe type ISE comprises a dialysis membrane 2 which is fixed to a glass tube 1 by a polytetrafluoroethylene tube 3. At a bottom of the glass tube, an ion-sensitive liquid 4 comprising dissolved nitrobenzene using a micro-syringe gently. Over the ion-sensitive liquid 4, an internal liquid 5 is poured, and a silver/silver chloride electrode 6 is dipped in the liquid 5 to assemble the film coated ISE. At an end of a slightly thick glass tube 7 in which the film coated ISE can be inserted, a polytetrafluoroethylene film 8 and a nylon mesh 10 as a spacer to hold the inner liquid are laminated. Over the end portion of the glass tube 7, a tetrafluoroethylene tube 11 is covered. Then, the film coated ISE is inserted in the glass tube 7 till it reaches the bottom of the glass tube 7. Between the glass tube 1 and the glass tube 7, an internal liquid 9 is poured. In the internal liquid 9, a silver/silver chloride electrode 6 wound around a glass tube is dipped. All the elements are fixed to prevent displacement of the elements. As understood, this electrode has a very complicated structure, and very high skill is required to assemble the polytetrafluoroethylene film, to pour the ion-sensitive liquid and the internal liquids. In addition, stability in time is not good. Therefore, since it is impossible to assemble a number of ISEs and store them, each ISE should be assembled when it is used. Thus, this ISE is not practically attractive.
Japanese Patent KOKAI Nos. 168045/1985, 88134/1986 and 175656/1987 disclose a method for solidifying the ion sensitive liquid, or a method for producing a minute film coated ISE. However, these methods cannot solve the troublesome assembling steps of the electrode or the necessity of skill. The handling property or the long-term stability of the produced ISE is not satisfactory. Further, there are still many problems to be solved relating to the product difference such as deviation of the resistance of the sensitive film or the response speed, the influence of the electrode surface or the electrode shape on the current, and the like.